Re: [PATCH v6 4/6] locking/pvqspinlock: Collect slowpath lock statistics

[Davidlohr Bueso]

On Mon, 14 Sep 2015, Waiman Long wrote:

> You can't use debugfs if we want to have per-cpu stats. We will have 
> to use sysfs instead. This will require more code changes. It is 
> certainly doable, but we have to choose between simplicity and 
> performance overhead. Right now, I am assuming that lock PV lockstat 
> is used primarily for debugging purpose and won't be enabled on 
> production system. If we want to have this capability in production 
> systems, we will certainly need to change it to per-cpu stats and use 
> sysfs instead.
>
> The original PV ticketlock code used debugfs and I was just following 
> its footstep. Do you think it is worthwhile to have this capability 
> available on production system by default?

If we can prove that the overhead is small enough, and do it correctly
(ie see how we do vmstats), it would be _very_ useful data to have
enabled by default for debugging performance issues; methinks. But right
now we have nowhere near that kind of data, not even with this atomic
variant -- although I recall you did mention a workload in a previous
iteration (which would be good to have in the changelog).
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